In a variety of manufacturing and production settings, there is a need to control alignment between various layers or within particular layers of a given sample. For example, in the context of semiconductor processing, semiconductor-based devices may be produced by fabricating a series of layers on a substrate, some or all of the layers including various structures. The relative position of these structures both within a single layer and with respect to structures in other layers is critical to the performance of the devices. The misalignment between various structures is known as overlay error.
The measurement of overlay error between successive patterned layers on a wafer is one of the most critical process control techniques used in the manufacturing of integrated circuits and devices. Overlay accuracy generally pertains to the determination of how accurately a first patterned layer aligns with respect to a second patterned layer disposed above or below it and to the determination of how accurately a first pattern aligns with respect to a second pattern disposed on the same layer. Presently, overlay measurements are performed via test patterns that are printed together with layers of the wafer. The images of these test patterns are captured via an imaging tool and an analysis algorithm is used to calculate the relative displacement of the patterns from the captured images. Such overlay metrology targets (or ‘marks’) generally comprise features formed in two layers, the features configured to enable measurement of spatial displacement between features of the layers (i.e., the overlay or displacement between layers). FIGS. 1A through 2B illustrate typical overlay targets of the prior art. FIGS. 1A and 1B illustrate overlay targets having 180 degree and 90 degree rotational symmetry, respectively, about a center of symmetry. Moreover, the target structures of FIGS. 1A and 1B include pattern elements (e.g., 102a through 108b), which are individually invariant to 90 degree rotation. Due to the 90 degree invariance of the individual pattern elements the pattern elements of targets 100 and 101 of FIGS. 1A and 1B are suitable for both X-overlay and Y-overlay measurements.
FIGS. 2A and 2B illustrate targets 200 and 201 which display invariance to a 90 degree and 180 degree rotation, respectively. In contrast to FIGS. 1A and 1B, the pattern elements (e.g., 202a through 208d) display only 180 degree rotational symmetry. As such, at least two separate orthogonally oriented pattern elements must be used in order to measure overlay in both the X- and Y-direction. For instance, the pattern elements 202a, 204a, 202d, and 204d may be used to measure overlay in a first direction, while elements 202b, 204b, 204c, and 202c may be used to measure overlay in a second direction orthogonal to the first direction.
Although existing targets and target measurement systems are suitable for many implementation contexts, it is contemplated herein that many improvements may be made. The invention described herein discloses targets and apparatus for enabling improved metrology measurements